Thermoplastic polymers

ABSTRACT

Thermoplastic polymers, for example fluoropolymers, are foamed by use of a solid formulation comprising thermoplastic polymer and manganese oxalate.

This invention relates to thermoplastic polymers and particularly,although not exclusively, relates to the foaming of high temperaturethermoplastic polymers. Preferred embodiments relate to high resistivitythermoplastic polymers which may be used in electrically insulatedproducts, such as cable insulation, cross-webs and tubing. In otherpreferred embodiments, high temperature thermoplastic polymers describedmay be used for tubing or stock profiles such as bar and rod.

Foaming of polymers is a known technique in the polymer industry for thelight-weighting of polymer components, improvement of electrical andthermal insulation properties, lower smoke generation, improvedflammability behaviour and improved strength to weight ratio. Inaddition there are clear benefits in terms of reduced material usage andtherefore lower cost for like parts when produced in foamed materialcompared to non-foamed components.

In general terms, there are two techniques used for the manufacture ofpolymer foams—physical and chemical foaming processes. Physical foamingof polymers is a process by which gas is injected directly into apolymer melt under high pressure during a processing operation. Theextent of foaming is controlled by a number of factors including thesolubility of the gas in the polymer, the use (or not) of nucleatingagents, the quantity of gas injected into the melt and the pressureunder which it is applied. The use of this type of foaming processrequires the use of specialised equipment. In contrast, chemical foamingagents can be used, the action of which is reliant on the thermaldegradation and breakdown of the foaming agent to produce a gas thatfoams the polymer.

Chemical foaming agents have already been used for a wide variety ofpolymer systems with considerable success but, thus far, efficient,thermally stable foaming agents for use with the highest processingtemperature polymers have remained elusive. Polymers that thus farremain difficult to foam using conventional foaming agents includefluoropolymers, for example fluorinated ethylene propylene copolymer(FEP), include perfluoroalkoxy (PFA), perfluoroalkoxy (methyl vinylether) (MFA) and ethylene tetrafluoroethylene (ETFE). Thus far chemicalfoaming agents based on ammonium polyphosphonate (EP 2065155) and mixedmetal carbonates/talc (US2009/0048359) have appeared in the literaturefor use in high temperature fluoropolymer applications. In the formercase the system based on ammonium polyphosphonate has seen littlecommercial use due to safety concerns associated with the use of ammoniaas the foaming gas while in the latter case little market knowledge isavailable.

In general terms, it is an object of the present invention to addressproblems associated with foaming of high performance thermoplastics.

According to a first aspect of the invention, there is provided a methodof preparing a foamed thermoplastic polymer which comprises subjecting amixture comprising an oxalate compound and a thermoplastic polymer to atemperature of greater than 300° C.

Said oxalate compound preferably includes at least one metal moiety.Preferably, it includes at least one transition metal moiety.Preferably, it includes a metal moiety with an atomic number of lessthan 42 or less than 37. The atomic number is suitably greater than 10or 18. Preferably, said oxalate compound includes a metal moietyselected from period IV of the Periodic Table. Said oxalate compoundpreferably includes a manganese moiety.

Preferably, the ratio of the weight of said at least one metal moietydivided by the total weight of metal moieties in said oxalate compoundis at least 0.4, preferably at least 0.6, more preferably at least 0.8,especially at least 0.95. Preferably, substantially the only metalmoiety in said oxalate compound is said one metal moiety. Thus, althoughoxalate compounds are known which are mixed metal compounds, saidoxalate compound preferably includes a single metal, which is preferablymanganese.

Preferably, the oxalate compound does not include any cationic orelectropositive moiety which is not a metal.

Said oxalate compound is preferably arranged to release a gas whensubjected to said temperature in the method. It is preferably arrangedto release a carbon-containing gas, for example carbon monoxide and/orcarbon dioxide. Said oxalate compound preferably does not release anitrogen-containing gas (e.g. ammonia) on heating.

Said oxalate compound may be hydrated. Suitably, however, it includesless than 40 wt %, preferably less than 30 wt % water. It preferablyincludes less than 10 wt %, less than 5 wt %, less than 2 wt %, lessthan 1 wt % or less than 0.1 wt % water. The oxalate compound mayinclude 1 ppm water. Inclusion of too large an amount of water in theoxalate compound could interfere with the foaming of the thermoplasticpolymer, since the water would be gaseous at a temperature far less than330° C. When a selected oxalate initially includes too large an amountof water, it may be treated to reduce the level of water prior to beingsubjected to said temperature of greater than 300° C. in the method.

Suitably, the ratio of the weight of said oxalate compound divided bythe total weight of all oxalate compounds in the mixture is at least0.6, preferably at least 0.8, more preferably at least 0.9. The ratio ispreferably 1. Thus, preferably only one type of oxalate compound isincluded in the mixture with said thermoplastic polymer.

Said oxalate compound may include 50 to 70 wt % of moiety C₂O₄,preferably 56 to 66 wt % of said moiety and especially 60 to 63 wt % ofsaid moiety.

Said oxalate compound may include 30 to 50 wt % of metal moieties,preferably 34 to 44 wt %, more preferably 37 to 40 wt %. Said metalmoieties are preferably selected from transition metals.

Said oxalate compound may include 25 to 40 wt %, preferably 30 to 40 wt%, especially 36 to 41 wt % of manganese moieties.

Said oxalate compound preferably comprises manganese oxalate whichsuitably includes less than 20 wt %, less than 10 wt %, or less than 5wt % water. Said oxalate compound preferably comprises 2 wt % or less or1 wt % or less of water.

Said oxalate compound preferably includes 36 to 41 wt % of manganesemoieties, 60 to 63 wt % of moiety C₂O₄ and 0 to 5 wt % of water,preferably 0 to 2 wt % of water. More preferably, said oxalate compoundsubjected to said temperature of greater than 300° C. includes less than1 wt % water.

Said oxalate compound is preferably particulate. Suitably, 100% of theparticles of said oxalate compound would pass through a sieve of meshsize 500 μm, preferably of mesh size 200 μm, more preferably of meshsize 100 μm, especially of mesh size 50 μm.

Said thermoplastic polymer is preferably a high performancethermoplastic polymer.

Unless otherwise stated, melting point of a polymer described herein maybe measured by DSC and the value at the top of the endothermic peakreported.

When said thermoplastic polymer is crystalline, it may have a meltingpoint of at least 250° C. It preferably has a melting point in the range250° C. to 320° C., for example 250° C. to 310° C.

Said thermoplastic polymer may have a continuous use temperature of atleast 160° C., preferably at least 190° C. Continuous use temperaturemay be determined in accordance with Underwriters' Laboratories Inc(UL), Standard for Polymeric Materials—Long Term Property Evaluators,UL746B. The continuous use temperature may be less than 300° C. or lessthan 265° C.

Said thermoplastic polymer may have a melt flow rate (MFR at 372° C./5.0kg) assessed using ISO12086 in the range 1.2 to 36 g/10 min, preferablyin the range 4 to 27 g/10 min, more preferably in the range 4-10 g/10min

Said thermoplastic polymer may have a tensile strength, measured inaccordance with ASTM D638 of at least 1500 psi, or preferably at least2000 psi. The tensile strength may be less than 8000 psi.

Said thermoplastic polymer may have a flexural modulus, in accordancewith ASTM D790 at +23° C. of at least 70,000 psi. The flexural modulusmay be less than 700,000 psi.

Said thermoplastic polymer may have a tensile modulus, in accordancewith ASTM D638, of at least 30,000, preferably at least 40,000. Thetensile modulus may be less than 600,000.

Said thermoplastic polymer may have a Tg of at least 125° C. or at least135° C.

Said thermoplastic polymer may have a heat distortion temperature,measured in accordance with D648, of at least 150° C., preferably atleast 170° C.

Said thermoplastic polymer suitably has a melt processing temperature ofat least 250° C. or at least 300° C.

Said thermoplastic polymer may be selected from fluoropolymers,high-performance polyamides (HPPAs), liquid crystal polymers,polyamideimides (PAls), polybenzimidazoles (PBIs), polybutyleneterephthalates (PBTs), polyetherimides (PEIs), polyimides (PIs),polyketones (PAEKs), polyphenylene sulfides (PPS), polysulfonederivatives, polycyclohexane dimethyl-terephthalates (PCTs) andsyndiotactic polystyrene.

Preferred fluoropolymers include FEP, PFA, MFA and ETFE. Preferredsulphones include PES, PPSE and PSMU. Preferred polyketones arepolyaryletherketones with polyetheretherketones (PEEK) being especiallypreferred.

In one embodiment, said thermoplastic polymer may be selected from PETand polycarbonate.

Said thermoplastic polymer is preferably a fluoropolymer and FEP isespecially preferred.

The ratio of the total weight of thermoplastic polymers divided by thetotal weight of oxalate compounds in said mixture may be less than 1000,suitably less than 700, preferably less than 500. Said ratio may be atleast 10, at least 50, or at least 90. The total amount of oxalatecompounds in said mixture may be in the range 0.1 to 2 wt % (e.g. 0.2 to1 wt %) and the total amount of thermoplastic polymers in said mixturemay be in the range 98 to 99.9 wt % (e.g. 99 to 99.8 wt %).

The ratio of the weight of said thermoplastic polymer divided by theweight of said oxalate compound in said mixture may be less than 1000,suitably less than 700, preferably less than 500. Said ratio may be atleast 10, at least 50 or at least 90. Said mixture may include 0.1 to 2wt % (e.g. 0.2 to 1 wt %) of said oxalate and 98 to 99.9 wt % (e.g. 99to 99.8 wt %) of said thermoplastic polymer.

The ratio of the wt % of said oxalate (preferably manganese oxalate)divided by the sum of the wt % of all blowing agents included in themixture (i.e. all blowing agents arranged to decompose to produce a gaswhich can foam the thermoplastic polymer) is suitably at least 0.8,preferably at least 0.9, more preferably at least 0.98, especially 1.

Said mixture preferably does not contain any protic acid. Said mixturepreferably does not include any phosphorous-containing blowing agent.

Said mixture preferably includes a single type of blowing agent, whichis suitably arranged to decompose to produce a gas which can foam thethermoplastic polymer. Said single type of blowing agent is preferablyarranged to release carbon monoxide and/or carbon dioxide and no othergas.

Said mixture may include a nucleator to facilitate formation of a foam.The nucleator is suitably not arranged to produce a gas duringpreparation of the foamed thermoplastic polymer. Said nucleator suitablycomprises (preferably consists of) inorganic particles. It may comprisetalc or a zeolite. A preferred nucleator is boron nitride.

In said mixture, the ratio of the total weight of oxalate compoundsdivided by the total weight of nucleators is suitably at least 0.5,preferably at least 0.9, more preferably at least 1. Said ratio may beless than 10, 8 or 5.

In said mixture, the ratio of the weight of said oxalate compounddivided by the weight of nucleators is suitably at least 0.5, preferablyat least 0.9, more preferably at least 1. Said ratio may be less than10. 8 to 5.

In said mixture, the ratio of the total weight of thermoplastic polymersdivided by the total weight of nucleators is suitably less than 1000,preferably less than 500. The ratio may be at least 20.

In said mixture, the ratio of the weight of said thermoplastic polymerdivided by the weight of nucleators is suitably less than 1000,preferably less than 500. The ratio may be at least 20.

A reference herein to “pbw” means “parts by weight”.

Preferably, said mixture includes said thermoplastic polymer, saidoxalate compound and said nucleator. Said mixture may include 0.1 to 2pbw (e.g. 0.20 to 1 pbw) total oxalate compounds, 0.1 to 2 pbw (e.g.0.15 to 0.8 pbw) total nucleators and 100 pbw in total of thermoplasticpolymers. Said mixture may include 0.1 to 2 pbw (e.g. 0.20 to 1 pbw) ofsaid oxalate compound, 0.1 to 2 pbw (e.g. 0.15 to 0.8 pbw) of saidnucleator and 100 pbw of said thermoplastic polymer. Said mixture mayinclude 0.20 to 0.80 pbw of said oxalate compound, 0.20 to 0.80 pbw ofsaid nucleator and 100 pbw of said thermoplastic polymer.

The sum of the pbw of all blowing agents in the mixture (i.e. allblowing agents arranged to produce a gas which can foam thethermoplastic polymer) is preferably 2 pbw or less, especially 1 pbw orless. The ratio of the pbw of said oxalate (preferably manganeseoxalate) divided by the sum of the pbw of all blowing agents in themixture is suitably at least 0.8, preferably at least 0.9, morepreferably at least 0.98, especially 1.

Preferably, said mixture includes 0.1 to 2 wt % (e.g. 0.2 to 1 wt %) ofsaid oxalate compound, 0.1 to 2 wt % (e.g. 0.15 to 0.8 wt %) of saidnucleator, and 96 to 99.8 wt % (e.g. 98.2 to 99.65 wt %) of saidthermoplastic polymer.

In the mixture, the sum of the wt % of manganese oxalate andthermoplastic polymer is preferably at least 98 wt %, more preferably atleast 99 wt %.

In the mixture, the sum of the wt % of manganese oxalate, thermoplasticpolymer and nucleators (especially boron nitride) is preferably at least98 wt %, more preferably at least 99 wt %.

Said method is preferably undertaken in an apparatus (e.g. a meltprocessing apparatus such as an extruder) operating at a maximumtemperature of less than 450° C. or less than 410° C.

In the method, said mixture may be subjected to a temperature of greaterthan 330° C., suitably greater than 340° C., preferably greater than350° C., more preferably greater than 360° C. It may be subjected to amaximum temperature of less than 390° C.

In the method, said mixture is suitably subjected to a temperature atwhich the oxalate compound breaks down to produce one or more gases, forexample carbon dioxide and/or carbon monoxide. The gas(es) producedsuitably cause(s) the polymer to foam.

The method is suitably carried out in a melt-processing apparatus, forexample an extruder or moulding apparatus such as an injection orrotational moulding apparatus. Preferably, the method is carried out inan extruder, for example a single or twin screw extruder.

In a first embodiment, said mixture comprising said oxalate compound andsaid thermoplastic polymer may be preformed. The method may include astep, prior to subjecting the mixture to a temperature of greater than300° C., of selecting a preformed mixture comprising said oxalatecompound and thermoplastic polymer, wherein said preformed mixture is ina solid state. For example, it may comprise pellets or granules of saidoxalate compound and thermoplastic polymer, said pellets or granulesbeing solid at Standard Ambient Temperature and Pressure (SATP) (i.e.25° C., 100 kPa). Said preformed mixture suitably includes oxalatecompound(s) and thermoplastic polymer(s) at the concentrations and/orratios described above. Thus, suitably in the method, the preformedmixture is selected and subsequently introduced into a melt processingapparatus, wherein it is subjected to a temperature (e.g. of 300° C. orhigher) as described above. The preformed mixture may optionally includea nucleator, for example boron nitride, at the concentrations and/orratios described above. Thus, in the method using the preformed mixture,no additional oxalate compound or thermoplastic polymer needs to beintroduced into the melt processing apparatus prior to the mixture beingsubjected to said temperature of greater than 300° C. in the preparationof said foamed thermoplastic polymer.

In a second embodiment, the method may comprise selecting athermoplastic polymer to be foamed in the method and contacting thethermoplastic polymer with a composition comprising said oxalatecompound. Initial contact may be prior to introduction of thermoplasticpolymer and oxalate compound into a melt processing apparatus or initialcontact may take place within a melt processing apparatus, for examplean extruder.

In an example of the second embodiment, said composition may comprise amasterbatch comprising said oxalate, which is suitably selected andcontacted with said thermoplastic polymer, suitably in a melt processingapparatus. Said masterbatch suitably comprises a thermoplastic polymerand said oxalate compound. The thermoplastic polymer in said masterbatchmay be the same type and/or have the same identity and/or besubstantially identical to the thermoplastic polymer to be foamed in themethod. Said masterbatch may include at least 5 wt %, preferably atleast 9 wt % of said oxalate compound. The total amount of oxalatecompounds in said masterbatch is preferably at least 5 wt % or at least9 wt %. Optionally, said masterbatch includes a nucleator; it mayinclude 0 to 10 wt % of a nucleator of the type described above.Preferably, said masterbatch includes 2 to 10 wt % of nucleator. Apreferred masterbatch includes 70 to 95 wt % of said thermoplasticpolymer, 5 to 30 wt % of said oxalate compound, (especially manganeseoxalate), 0 to 10 wt % of said nucleator (preferably boron nitride) and0 to 20 wt % other materials. Other materials may be selected fromcolours, anti-oxidants and/or other additives to improve ageingproperties of the foamed thermoplastic polymer such as zinc oxide. Saidmasterbatch preferably include 80 to 95 wt % of said thermoplasticpolymer, 5 to 20 wt % of manganese oxalate and 0 to 10 wt % of saidnucleator which is preferably boron nitride.

In the method, said masterbatch may be contacted with said thermoplasticpolymer to be foamed at a let down ratio in the range 5:1 to 30:1.Suitably, 1 to 36 wt % of said masterbatch is contacted with 64 to 99 wt% of said thermoplastic polymer to be foamed in the method, contactsuitably taking place in a melt processing apparatus, preferably anextruder. For example, pellets of the masterbatch and pellets of thethermoplastic polymer may be introduced into an extruder via itsfeedthroat and the two components melt processed together to form amixture which foams in the method. Suitably 3 to 30 wt %, preferably 5to 15 wt % of said masterbatch is contacted with 70 to 97 wt %,preferably 85 to 95 wt % of said thermoplastic polymer to be foamed inthe method.

Said mixture preferably includes less than 2 wt %, less than 1 wt % orless than 0.5 wt % water immediately prior to being subjected to saidtemperature greater than 300° C.

Pellets of the masterbatch of the second embodiment suitably includeless than 1 wt %, preferably less than 0.5 wt %, especially less than0.1 wt % water immediately prior to contact with said thermoplasticpolymer.

In the method, the foamed thermoplastic polymer suitably has a voidcontent (measured as described herein) of at least 25%, preferably atleast 35%, more preferably at least 45%. The void content may be lessthan 70% or less than 60%. The foamed thermoplastic polymer may have adensity in the range 0.6 to 1.7 g/cm³.

In the method, said foamed thermoplastic polymer may define a productwhich is suitably formed by extrusion. Said product may include anopening surrounded by said foamed thermoplastic polymer. Said productmay be elongate (suitably having a length of at least 1 m or at least 10m). Such a product may have a cross-section, taken transverse to theaxis of elongation, comprising an opening defined by said foamedthermoplastic polymer. Said product may comprise cable insulation,cross-webs or tubing. It may comprise a stock shape, such as bar or rod.Said product may be associated with an electrical conductor; andsuitably the foamed thermoplastic polymer defines an insulation materialfor the electrical conductor. Thus, the method suitably comprisesassociating an electrical conductor (which is suitably arranged toconvey current between two locations) with said foamed thermoplasticpolymer.

The foamed thermoplastic polymer may be used in products having improvedflame or smoke properties or reduced weight. The foamed thermoplasticpolymer may have an improved strength to weight ratio compared tonon-foamed thermoplastic polymer.

According to a second aspect of the invention, there is provided aformulation for use in preparing a foamed thermoplastic polymer, forexample in the method of the first aspect, said formulation comprising athermoplastic polymer and an oxalate compound as described according tosaid first aspect.

Said oxalate compound and thermoplastic polymer may be as describedaccording to the first aspect.

Said formulation preferably includes less than 2 wt %, less than 1 wt %,less than 0.5 wt % or less than 0.1 wt % of water. The formulation mayinclude at least 1 ppm water.

Said formulation preferably comprises a 0.2 to 30 pbw of said oxalatecompound and 70 to 99.8 pbw of said thermoplastic polymer. In a firstembodiment, wherein said formulation need not be let down in amelt-processing apparatus for use in preparing a foamed thermoplasticpolymer, said formulation may comprise 0.2 to 1 pbw (preferably 0.2 to0.5 pbw) of said oxalate and 99.0 to 99.8 pbw (preferably 99.5 to 99.8pbw) of said thermoplastic polymer. In a second embodiment, saidformulation may comprise a solid masterbatch which may be let down inthe method. In this case, the masterbatch may include 1 pbw to 30 pbw(suitably 3 to 15 pbw, preferably 5 to 15 pbw) of said oxalate compoundand 70 to 99 pbw (suitably 85 to 97 pbw, preferably 85 to 95 pbw) ofsaid thermoplastic polymer.

Said formulation preferably comprises 0.2 to 30 wt % of said oxalatecompound and 70 to 99.8 wt % of said thermoplastic polymer. In saidfirst embodiment, said formulation comprises 0.2 to 1 wt % (preferably0.2 to 0.5 wt %) of said oxalate compound and 99.0 to 99.8 wt %(preferably 99.5 to 99.8 wt % of said thermoplastic polymer. In saidsecond embodiment, said formulation may comprise 1 to 30 wt %(preferably 5 to 15 wt %) of said oxalate compound and 70 to 99 wt %(preferably 85 to 95 wt %) of said thermoplastic polymer.

Said formulation is preferably a solid having a melting point of atleast 250° C. Said formulation may include a nucleator.

When said formulation includes a nucleator, it may include 0.2 to 30pbw, for example 3 to 15 pbw, of said nucleator, for example boronnitride.

Said formulation may be in the form of pellets. Preferably, said pelletsare homogenous. Said pellets may have a volume in the range 2 to 50 mm³,suitably 4 to 30 mm³. They suitably include said thermoplastic polymer,said oxalate compound and said nucleator. Said pellets may include 1 to11 pbw (e.g. 2 to 8 pbw) total oxalate compounds, 0 to 11 pbw (e.g. 1 to10 pbw) total nucleators and 100 pbw in total of thermoplastic polymers.Said pellets may include 1 to 11 pbw (e.g. 2 to 8 pbw) of said oxalatecompound (e.g. manganese oxide), 0 to 11 pbw (e.g. 1 to 11 pbw) of saidnucleator and 100 pbw of said thermoplastic polymer.

The sum of the pbw of all blowing agents in the pellets (i.e. allblowing agents arranged to produce a gas which can foam thethermoplastic polymer) is preferably 10 pbw or less, especially 8 pbw orless based on 100 pbw in total of thermoplastic polymers in the pellets.The ratio of the pbw of said oxalate (preferably manganese oxalate)divided by the sum of the pbw of all blowing agents in the pellets issuitably at least 0.8, preferably at least 0.9, more preferably at least0.98, especially 1.

Preferably, said pellets include 1 to 10 wt % (e.g. 3 to 9 wt %) of saidoxalate compound, 0 to 10 wt % (e.g. 2 to 10 wt %) of said nucleator,and 89 to 99 wt % (e.g. 87 to 95 wt %) of said thermoplastic polymer.

In the pellets, the sum of the wt % of manganese oxalate andthermoplastic polymer(s) is preferably at least 98 wt %, more preferablyat least 99 wt %.

In the pellets, the sum of the wt % of manganese oxalate, thermoplasticpolymer(s) and nucleator(s) (especially boron nitride) is preferably atleast 98 wt %, more preferably at least 99 wt %.

Preferred thermoplastic polymers may be selected from fluoropolymers,high-performance polyamides (HPPAs), liquid crystal polymers,polyamideimides (PAIs), polybenzimidazoles (PBIs), polybutyleneterephthalates (PBTs), polyetherimides (PEIs), polyimides (PIs),polyketones (PAEKs), polyphenylene sulfides (PPS), polysulfonederivatives, polycyclohexane dimethyl-terephthalates (PCTs) andsyndiotactic polystyrene.

The preferred specified groups of polymers have a melting point in therange 250 to 320° C. or in the range 250 to 310° C.

Preferred fluoropolymers include FEP, PFA, MFA and ETFE. Preferredsulphones include PES, PPSE and PSMU. Preferred polyketones arepolyaryletherketones with polyetheretherketones (PEEK) being especiallypreferred.

In one embodiment, said thermoplastic polymer may be selected from PETand polycarbonate.

Said thermoplastic polymer is preferably a fluoropolymer and FEP isespecially preferred.

Said oxalate may be as described according to the first aspect. It ispreferably manganese oxalate.

Said formulation preferably comprises a thermoplastic polymer which is afluoropolymer (e.g. FEP) and manganese oxalate. In one preferredembodiment, said formulation comprises 0.2 to 1 wt % of manganeseoxalate and 99.0 to 99.8 wt % of fluoropolymer, for example FEP. Inanother preferred embodiment, said formulation comprises 1 to 30wt % ofmanganese oxalate and 70 to 99 wt % of fluoropolymer, for example FEP.According to a third aspect of the invention, there is provided a methodof making a formulation according to the second aspect, the methodcomprising selecting an oxalate compound and selecting a thermoplasticpolymer and contacting said oxalate compound with said thermoplasticpolymer.

Pellets comprising oxalate compound and thermoplastic polymer may beprepared.

In the method, preferably the temperature of the components (e.g.oxalate and thermoplastic polymer) does not rise above 330° C. andpreferably does not rise above 300° C., for example prior to formationof said pellets. The formulation prepared may be in solid form, forexample in the form of pellets; it may comprise a solid masterbatch or asolid formulation which is preformed with appropriate levels of oxalateand thermoplastic polymer for foaming in the method of the first aspect,without being let down. In the method of making said formulation, amixture comprising said oxalate and thermoplastic polymer may be treatedin a melt processing apparatus, for example an extruder. The mixture ispreferably heated to a temperature of less than 330° C. or morepreferably less than 300° C. in the melt processing apparatus. Forexample, when the melt processing apparatus comprises an extruder, themaximum temperature of zones of the extruder to which the mixture issubjected is suitably less than 330° C. or less than 300° C. Thus, inthe method for making said formulation, the oxalate is not subjected toa temperature of greater than 300° C. and, accordingly, it does notsignificantly decompose to release any carbon-containing gas. However,advantageously, water of hydration (if any) associated with the oxalatecompound may be driven off during formation of the solid form.Consequently, when the solid form is used in the method of the firstaspect, there is negligible water contained within the oxalate compoundwhich might otherwise interfere with the foaming process.

According to a fourth aspect of the invention, there is provided afoamed thermoplastic polymer comprising the residue of an oxalatecompound, for example the residue remaining after the method of thefirst aspect. The residue may comprise a said metal moiety as describedaccording to the first aspect. The preferred residue comprises an oxidecomprising said metal moiety. A preferred residue is manganese oxide.The aforementioned residue may be present at less than 1 wt %, forexample at 0.01 to 1 wt % in said foamed plastic polymer.

The foamed thermoplastic polymer may be selected from fluoropolymers,high-performance polyamides (HPPAs), liquid crystal polymers,polyamideimides (PAls), polybenzimidazoles (PBIs), polybutyleneterephthalates (PBTs), polyetherimides (PEIs), polyimides (PIs),polyketones (PAEKs), polyphenylene sulfides (PPS), polysulfonederivatives, polycyclohexane dimethyl-terephthalates (PCTs) andsyndiotactic polystyrene.

The preferred specified groups of polymers have a melting point in therange 250 to 320° C. or in the range 250 to 310° C.

Preferred fluoropolymers include FEP, PFA, MFA and ETFE. Preferredsulphones include PES, PPSE and PSMU. Preferred polyketones arepolyaryletherketones with polyetheretherketones (PEEK) being especiallypreferred.

In one embodiment, said thermoplastic polymer may be selected from PETand polycarbonate.

Said thermoplastic polymer is preferably a fluoropolymer and FEP isespecially preferred.

Said foamed thermoplastic polymer preferably includes at least 90 wt %of said thermoplastic polymer.

Said foamed thermoplastic polymer with said residue may be in the formof a product as described in the first aspect.

Said foamed thermoplastic polymer may include a nucleator as describedabove, suitably at a level of 0.01 to 1 wt %.

Any aspect of any invention described herein may be combined with anyfeature of any other invention or embodiment described herein mutatismutandis.

Specific embodiments of the invention will now be described, by way ofexample.

The following materials are referred to hereinafter:

Manganese oxalate refers to manganese (II) oxalate dihydrate obtainedfrom Alfa Aesar GmbH.

DuPont FEP106—refers to fluorinated ethylene propylene copolymersobtained from DuPont.

DuPont FEPCJ99—refers to fluorinated ethylene propylene copolymersobtained from DuPont.

DuPont FEP9494—refers to fluorinated ethylene propylene copolymersobtained from DuPont.

SI64.1 nucleator—refers to a masterbatch containing 5 wt % boron nitride(BN) obtained from Colorant Chromatics Group.

EXAMPLE 1 Manufacture of Manganese Oxalate Masterbatch Concentrate

Manganese oxalate was pre-blended with DuPont FEP106 pellets by tumblemixing. Tumble blends containing both 5 wt % and 10 wt % manganeseoxalate were produced. The pre-blended mixture was then fed byvolumetric feeder into a 30mm diameter twin screw extruder with let-downratio of 22/1. The extruder temperature settings were255/255/260/260/265/270/270° C. from the feed throat to the dierespectively. The extruder is vented to allow the removal of water ofhydration during the masterbatch manufacture process as this has thepotential to cause premature foaming in subsequent use of the materialif not removed. Improved dispersion of the manganese oxalate wasobserved after multiple extruder passes. The masterbatch is pelletisedat the end of the process.

EXAMPLE 2 Manufacture of Manganese Oxalate Masterbatch Concentrate withAlternative Resin

Manganese oxalate was pre-blended with DuPont CJ99 pellets by tumblemixing. A tumble blend containing 5% wt manganese oxalate was produced.The pre-blended mixture was then fed by volumetric feeder into a 30 mmdiameter twin screw extruder with length/diameter ratio of 22/1. Theextruder temperature settings were 276/290/280/280/278/282/285° C. fromthe feed throat to the die respectively. The extruder is vented to allowthe removal of water of hydration during the masterbatch manufactureprocess as this has the potential to cause premature foaming insubsequent use of the material if not removed. The dispersion of themanganese oxalate was improved after multiple passes through theextruder under these conditions. The masterbatch is pelletised at theend of the process.

EXAMPLE 3 Manufacture of a Combined Manganese Oxalate/Boron NitrideConcentrate

Manganese oxalate was pre-blended with DuPont CJ99 pellets by tumblemixing. A tumble blend containing 5.5% wt manganese oxalate wasproduced. The pre-blended mixture was then fed by volumetric feeder intoa 30 mm diameter twin screw extruder with length/diameter ratio of 22/1.The extruder temperature settings were 276/290/280/280/278/282/285° C.from the feed throat to the die respectively. The resulting masterbatchwas then pre-blended with 5 wt % boron nitride and extruded as a secondstep using exactly the same processing conditions as the previous step.The resultant formulation therefore contains 5.225% wt manganese oxalateand 5% wt boron nitride. The masterbatch is pelletised at the end of theprocess.

EXAMPLE 4 Measurement of Foaming

The measurement of the density of the produced foams is carried outusing an AG 104 density balance manufactured by Mettler Toledo. Thesample is weighed both in air and in water to obtain the density of thematerial under investigation. The resultant density is used in thecalculation of the void content as follows

Void content=100−((p1/p2)*100)

where p1 and p2 are the densities of the foamed polymer and theunmodified polymer respectively.

EXAMPLE 5 Manufacture of Foamed Samples

The manganese oxalate masterbatch was tumble blended with other pellets(e.g. nucleator and/or diluent resin) to be used in the formulationstested. The blend was then added at the feed throat of a single screwextruder with a temperature profile of 280/300/330/380/360/360/340° C.from the feed throat to the die as this is representative of processingconditions used in the manufacture of wire and cable and other FEPproducts. Different screw speeds were assessed in addition to the effectof the presence of nucleating agent (S164.1) or not. The processingconditions and results are summarised in Table 1. The diluent resin inall cases was DuPont FEP 106.

TABLE 1 Loading of foaming Loading of masterbatch nucleator Void ofExample 1 masterbatch Screw Density content Sample (wt %) (wt %) rpm(g/cm3) (%) 1   5% 5% 10 1.36 37.222 2a 10% — 18 1.06 50.972 3b 10% — 181.01 53.241 3a 10% 5% 18 1.09 49.722 3b 10% 5% 18 1.06 50.926

EXAMPLE 6 Manufacture of Foamed Samples Using Pellets from Example 2

The manganese oxalate masterbatch was tumble blended with other pellets(e.g. nucleator and/or diluent resin) to be used in the formulationstested. The blend was then added at the feed throat of a single screwextruder with different temperature profiles(T1=280/300/350/400/390/380/380° C.; T2=280/300/350/400/380/370/370 andT3=290/310/360/400/390/380/380° C. from the feed throat to the die) inorder to find the optimum processing temperature. Different screw speedswere assessed in addition to the effect of the presence of nucleatingagent (S164.1) or not. The processing conditions and results aresummarised in Table 2. The diluent resin in all cases was DuPont FEPCJ99.

TABLE 2 Loading of foaming masterbatch Loading of of nucleator Temp VoidExample 2 masterbatch pro- Screw Density content Sample (wt %) (wt %)file rpm (g/cm3) (%) 4a 5% — T1 10 1.91 11.136 4b 5% — T1 20 1.89 12.0934c 5% — T1 30 1.86 13.488 4d 5% — T1 40 1.84 14.419 4e 5% — T1 50 1.7020.930 4f 5% — T3 20 1.82 15.35 4g 5% — T3 40 1.84 14.42 4h 5% — T3 501.49 30.70 5a 10% — T3 20 1.58 26.51 5b 10% — T3 40 1.35 37.21 6a 5% 5%T3 20 1.26 41.40 6b 5% 5% T3 40 1.20 44.19 7a 10% 2.5%   T3 10 1.6224.65 7b 10% 2.5%   T3 20 1.45 32.56 7c 10% 2.5%   T3 40 0.97 54.88 8a10% 5% T3 20 1.21 43.72 8b 10% 5% T3 40 1.03 52.09

EXAMPLE 7 Manufacture of Foamed Samples Using Pellets from Example 3

The combined masterbatch containing both manganese oxalate and boronnitride produced in example 3 was tumble blended with diluent resinpellets to produce the formulations tested. The blend was then added tothe feed throat of a single screw extruder using the temperature profileT3 as described in example 6 (i.e. T3=290/310/360/400/390/380/380° C.from the feed throat to the die). Different screw speeds were assessedand the diluent resin in all case was DuPont FEP CJ99. The results aresummarised in Table 3.

TABLE 3 Loading of foaming masterbatch Void of Example Screw Densitycontent Sample 3 (wt %) rpm (g'cm3) (%)  9a  5% 20 1.57 26.98  9b  5% 401.25 41.86  9c  5% 50 1.08 49.77 10a 10% 20 1.17 45.58 10b 10% 40 0.8958.60 10c 10% 50 0.77 64.19From these results the synergistic effect of manufacturing themasterbatch with the incorporation of a single type of masterbatchpellet comprising both manganese oxalate and the boron nitridenucleating agent is evident. For example, in example 6 samples 6a and 6bare analogous to samples 10a and 10b in example 7. In examples 10a and10b the density is reduced compared to similar samples produced byutilising separate nucleator and foaming agent masterbatches. Inaddition, sample 9b indicates that using half the active ingredient canresult in similar density reductions when a combined masterbatchapproach is made.

EXAMPLE 8 Manufacture of a Combined Manganese Oxalate/Boron NitrideMasterbatch by an Alternative Method

Dupont CJ99 resin was tumble blended with manganese oxalate and boronnitride (each at concentrations of 5 wt %). The resultant mixture wasthen fed by volumetric feeder into a 30 mm diameter twin screw extruderwith length/diameter ratio of 22/1. The extruder temperature settingswere 276/290/280/280/278/282/285° C. from the feed throat to the dierespectively. The extruder is vented to allow the removal of water ofhydration during the masterbatch manufacture process as this has thepotential to cause premature foaming in subsequent use of the materialif not removed. The dispersion of the additives was improved aftermultiple passes through the extruder under these conditions. Themasterbatch is pelletised at the end of the process.

EXAMPLE 9 Manufacture of Foamed Wire from Pellets Of Example 8

The combined masterbatch containing both manganese oxalate and boronnitride produced in example 8 was tumble blended with diluent resinpellets (Dupont FEP 9494) at a ratio of 10% wt. the resultantcomposition was extruded onto 24 AWG bare copper using a 38 mm singlescrew extruder with L/D ratio of 24/1 and a standard FEP screw. Theextruder was fitted with a 6.35 mm die with a 1.5 mm tip. A temperatureprofile of 290/310/370/400/405/405/410/410/425 was used from tip to diewith a screw speed of 34 rpm. The resultant insulation had a density of1.262 g/cm3 and a void content of 41%.

Thus, it is clear from the experiments undertaken that the manganeseoxalate masterbatch can advantageously be used to foam the highperformance (high melting) thermoplastic. Other high meltingthermoplastics may be foamed in a similar manner. i.e. by themanufacture of a masterbatch/compound containing the manganese oxalatefoaming agent or a masterbatch/compound containing manganese oxalatefoaming agent and boron nitride nucleating agent.

As an alternative to the use of a masterbatch as described, a mixturecomprising manganese oxalate (e.g. 0.25-2 wt %) and thermoplasticpolymer may be made, in a manner analogous to that described in Example1, except that the mixture can be used directly in manufacturing afoamed sample without needing to be diluted. Such a mixture could alsoincorporate nucleator (e.g. boron nitride) at an appropriateconcentration (e.g. 0.25 to 2 wt %) so the thermoplasticpolymer/manganese oxalate/boron nitride mixture can be used directly tomake foamed products.

The invention is not restricted to the details of the foregoingembodiment(s). The invention extends to any novel one, or any novelcombination, of the features disclosed in this specification (includingany accompanying claims, abstract and drawings), or to any novel one, orany novel combination, of the steps of any method or process sodisclosed.

1. A formulation for use in preparing a foamed thermoplastic polymer,said formulation comprising a thermoplastic polymer and an oxalatecompound.
 2. (canceled)
 3. A formulation according to claim 1, whereinsaid oxalate compound comprises manganese oxalate.
 4. A formulationaccording to claim 3, wherein said formulation includes less than 0.5 wt% water.
 5. A formulation according to claim 3, which includes 0.2 to 30parts by weight (pbw) of said oxalate compound and 70 to 99.8 pbw ofsaid thermoplastic polymer.
 6. A formulation according to claim 3, whichincludes a masterbatch which includes 1 pbw to 30 pbw of said oxalatecompound and 70 to 99 pbw of said thermoplastic polymer.
 7. Aformulation according to claim 3, wherein said formulation is a solidhaving a melting point of at least 250° C.
 8. A formulation according toclaim 3, wherein said formulation is in the form of pellets which arehomogenous, wherein said pellets include 1 to 11 pbw total oxalatecompounds, 0 to 11 pbw total nucleators and 100 pbw in total ofthermoplastic polymers.
 9. (canceled)
 10. A formulation according toclaim 8 wherein the sum of the pbw of all blowing agents in the pelletsis 10 pbw or less based on 100 pbw in total of thermoplastic polymers.11. A formulation according to claim 3, wherein said pellets include 1to 10 wt % of said oxalate compound, 0 to 10 wt % of said nucleator and89 to 99 wt % of said thermoplastic polymer.
 12. A formulation accordingto claim 3, wherein, in said pellets, the sum of the wt % of manganeseoxalate and thermoplastic polymer(s) is at least 98 wt %.
 13. (canceled)14. A formulation according to claim 3, wherein said thermoplasticpolymer is a fluoropolymer.
 15. A method of preparing a foamedthermoplastic polymer which comprises subjecting a mixture comprising anoxalate compound and a thermoplastic polymer to a temperature of greaterthan 300° C. 16.-18. (canceled)
 19. A method according to claim 15wherein said thermoplastic polymer to be foamed is a fluoropolymer andthe method comprises: selecting a masterbatch comprising saidthermoplastic polymer and manganese oxalate; contacting said masterbatchwith said thermoplastic polymer; and subjecting a mixture comprisingsaid masterbatch and said thermoplastic polymer to a temperature ofgreater than 300° C.
 20. A method according to 19, wherein saidthermoplastic polymer to be foamed is FEP.
 21. (canceled)
 22. A methodaccording to claim 19, wherein the total amount of oxalate compounds insaid mixture is in the range 0.1 to 2 wt % and the total amount ofthermoplastic polymers in said mixture is in the range 98 to 99.9 wt %.23.-27. (canceled)
 28. A method according to claim 19, wherein 1 to 36wt % of said masterbatch is contacted with 64 to 99 wt % of saidthermoplastic polymer to be foamed in the method.
 29. A method of makinga formulation according to claim 1, the method comprising selecting anoxalate compound and selecting a thermoplastic polymer and contactingsaid oxalate compound with said thermoplastic polymer.
 30. (canceled)31. A method according to claim 29 wherein, in the method, thetemperature of the components does not rise above 330° C. prior toformation of pellets comprising said oxalate compound and thermoplasticpolymer, wherein said oxalate compound is manganese oxalate. 32.(canceled)
 33. A formulation according to claim 1, said formulationcomprising a solid masterbatch which comprises a thermoplastic polymerand manganese oxalate, wherein said formulation has a melt processingtemperature of at least 250° C., wherein said thermoplastic polymer is afluoropolymer, wherein said formulation comprises 5 to 15 parts byweight of manganese oxalate and 85 to 95 parts by weight of saidthermoplastic polymer.
 34. A formulation according to claim 3, whereinsaid thermoplastic polymer is FEP.